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Greetings,
I'd like to run this idea by the fertile minds here at the OLF. I've used the Cumpiano joint on all my guitars with no known failures yet but have heard of some reported.
I'm guessing it's a matter of time. Bill C has an idea for reinforcement on his website. Here's another. 1/8 x 3/8 carbon bars to toughen the front edge of the holes where I'm assuming the stress would be. Tests on mock up tenons were OK so I'm currently trying one on a real guitar. Jim Watts turned me onto a good source of carbide drill bits so I don't ruin regular ones drilling this stuff. Anyone see any pitfalls I may have missed or not thought of before I forge ahead?
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I think that looks like a great idea. My only thing would be to wonder if carbon fiber is the right material for this. I have no experience so this is more of my own question on the approach. But my thinking is that carbon fiber works for load and when it fails, it sort of shatters. Would the stress point possibly cause the circular hole in the CF cause an isolated stress point and lead to shattering in the CF? Of course wood would probably go first if that amount of force is put on.

So again. I really don't know what I am saying and may be off on the entire theory. I like the idea.

John

Hi Terry,

I have been casing the entire tenon in a separate wood. I glue a patch on the front first and then the sides after. I have been using rosewood offcuts and making them around 1/16. On the last few I have cut a little into the heel when making the tenon (I cut mine on the table saw) so the side support pieces extend a little into the heel also. I see it as insurance against a crack really opening up as it would have to crack through separate cuts of wood. I did have an early one crack on me but I didn't have nearly enough meat on the tenon in the front. With the reinforcements glued on I have been able to make the tenon smaller also, down to 5/8 or less sometimes so I can make the heel slimmer. Since the CF is not in compression where you are using it I wonder if it needs to be CF? Of course I am not the most scientifically minded so it may be exactly the right thing.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

The last neck joint i did used the cumpiano style joint with the crossgrain hardwood slats on either side of the tenon like he describes in the update on his website. I am thinking it will be "enough", and it doesnt add much of any time or effort.

I sort of think that the hardwood slat method may have an advantage, in that it has a larger glueing surface than the CF. Either way is probably plenty good, but my totally unsupported theory is that a tenon with CF may be pulled foreward before one with crossgrain slats crack.

Burtons entirely cased tenon seems really good, and i also like to extend the side supports into the heel a little.

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Jordan Aceto
Ithaca, NY

Would this be much of a problem if you use laminated neck?

Also, wouldn't putting 1/2" dowel in the tenon and putting slats on the either side will be more than enough?

Thanks for the input guys. Much appreciated. Not using wood slat reinforcement was kind of laziness on my part as I use a Fox type router jig to do the tenon and figured I'd have to make a new template to do a narrower tenon so I could use the slats. I imagine the whole tenon could theoretically pull off behind the bars with my setup. I'll have to do some torture tests on the trial tenons to see how they fail. The wood reinforcement may be better.
Thanks Again
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

You don't need a new template to do a narrower tenon. Use a different template guide or a bigger bearing on your bit.
FWIW I use a bolted dovetail (with a bolt on fingerboard extension). I plan on doing a tutorial on it soon. I've got to make pictures while I'm doing one. With this type joint there's still no glue, but the bolt isn't stressed like it is when they actually pull the neck into the body. All the bolt has to do is hold the neck down. The cheeks on the dovetail hold it in.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Ahhh, right, i did it on the tablesaw, and just slid the slats in with glue on one face like cumpiano shows in the update. If i had to actually DO SOMETHING, i would probably get lazy too!

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Jordan Aceto
Ithaca, NY

where do you find compiano's update?

http://www.cumpiano.com/Home/Articles/S ... block.html

About halfway down the page, it says "NEW! FURTHER IMPROVEMENT..."

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Jordan Aceto
Ithaca, NY